Refractive optics requires materials having high transmittance. For semi-conductor applications where smaller and smaller features are desired at the 248 and 193 nm wavelengths, high purity fused silica has been shown to exhibit the required minimum transmittance of 99%/cm or better.
Projection optical photolithography systems that utilize the vacuum ultraviolet wavelengths of light below 193 nm provide benefits in terms of achieving smaller feature dimensions. Such systems that utilize vacuum ultraviolet wavelengths in the 157 nm wavelength region have the potential of improving integrated circuits with smaller feature sizes. Current optical lithography systems used by the semiconductor industry in the manufacture of integrated circuits have progressed towards shorter wavelengths of light, such as the popular 248 nm and 193 nm wavelengths, but the commercial use and adoption of vacuum ultraviolet wavelengths below 193 nm, such as 157 nm has been hindered by the transmission nature of such vacuum ultraviolet wavelengths in the 157 nm region through optical materials. Such slow progression by the semiconductor industry of the use of VUV light below 175 nm such as 157 nm light has been also due to the lack of economically manufacturable photomask blanks from optically transmissive materials. For the benefit of vacuum ultraviolet photolithography in the 157 nm region such as the emission spectrum VUV window of a F.sub.2 excimer laser to be utilized in the manufacturing of integrated circuits there is a need for mask blanks that have beneficial optical properties including good transmission below 164 nm and at 157 nm and that can be manufactured economically.
The present invention overcomes problems in the prior art and provides a economical high quality improved photomask blanks and lithography glass that can be used to improve the manufacturing of integrated circuits with vacuum ultraviolet wavelengths.
Use of high purity fused silica as optical elements in photolithography stems from the fact that high purity fused silica is transparent over a wide range of wavelengths, spanning from the infrared to deep ultraviolet regions. Furthermore, high purity fused silica exhibits excellent chemical durability and dimensional stability. These properties have made high purity fused silica highly suited for use as optical lenses as well as for photomask substrates in photolithography, but use has been limited to the KrF and ArF wavelength regions.
Photomask glass qualifications are comparatively different from other optical elements used in photolithography in that they characteristically have smaller thicknesses of as low as only a few millimeters through the optical path. As such, they must meet very strict requirements for dimensional stability (warping and shrinkage) in order to ensure the extreme accuracy required to form fine circuit patterns on the photomask plate and target. And as the demand for even smaller features continues to drive the lasing wavelength further down to the 157 nm region and lower, the choice of optical materials meeting the minimum required transmittance becomes severely limited for all optical elements, but even more so for photomask substrates due to the reasons stated above. Crystalline materials such as calcium fluoride, barium fluoride and magnesium fluoride for example, have been shown to exhibit transmittances which are suitable for 157 nm wavelength applications. Unfortunately, these materials tend to have certain drawbacks making them unsuitable for these applications, in addition to manufacturing/economic problems. For example, calcium fluoride exhibits unacceptably high thermal expansion properties for photomask applications in the 157 nm wavelength region. Magnesium fluoride on the other hand, exhibits acceptable expansion but is unsuitable because it is naturally birefringent.
It has been suggested in EP 0 636 586 A1 that in order to be suitable for use as a photomask substrate for certain photolithography applications at 248 and 193 nm wavelengths, high purity fused silica made by the direct flame method must contain high molecular hydrogen in the range of 10.sup.17 to 10.sup.19 molecules/cm.sup.3. Similarly, JP 1-201664 discloses that synthetic quartz glass for use as photomask material whose optical properties have been changed due to sputtering, plasma etching or excimer irradiation, can be restored to its original condition by heat treating the glass in a hydrogen atmosphere. Specifically, this document describes the effect on synthetic quartz of exposure to 248 and 193 nm wavelengths. The effect of exposure to 248 and 193 nm wavelengths on fused silica is also described in "Densification of Fused Silica under 193 nm excitation," by Borrelli et al, in J. Opt. Soc. Am. B/Vol. 14, No. 7, pp. 1606-1615 (July 1997); and by Allan et al., in "193-nm excimer-laser-induced densification of fused silica," Optics Letters, Vol. 21, No. 24, pp. 1960-1962 (Dec. 5, 1996).
EP 0 901 989 A1 discloses a manufacturing method for making silica glass substantially free of chlorine. In a direct deposit concurrent vitrifying process silicon tetrafluoride is flame hydrolyzed to provide a silica glass in which fluorine is controlled within the range 100 ppm to 450 ppm and OH group density in the range from 600 ppm to 1300 ppm.
U.S. Pat. No. 5,326,729 discloses quartz glass having excimer laser resistance produced by subjecting the glass to dehydration treatment in a temperature range lower than the transparent vitrification temperature of the glass followed by transparent vitrification and molding to a desired shape, followed by a doping treatment in a hydrogen atmosphere.
U.S. Pat. No. 5,474,589 discloses a UV light permrable fluorine-doped synthetic quartz glass with decreased defects.
Applicants, previously have disclosed several effective methods for improving the optical properties of high purity fused silica when used as an optical lens in photolithography at both the 248 and 193 nm wavelength regions. See for example, U.S. Pat. Nos. 5,616,159; 5,668,067 and 5,735,921 all incorporated herein by reference.
Accordingly, it is an object of the present invention to disclose a silicon oxyfluoride glass for use as a photomask substrate at VUV wavelengths below 193 nm, preferably in the F.sub.2 Excimer Laser 157 nm region, method of making such glass, and method for characterizing such silicon oxyfluoride glass.